Hello
How can I control voltage level on a specific pin in PIC18F? Let's say then I want to send about 0.6V to transistor base. But I cannot use a voltage divider. I need to control output voltage by my PIC18F (PIC18F2525)
Is it a programming issue?

There is a threshold voltage, anything above will get the transistor working, so a transistor won't care if it's 5v or 0.6v, as long as there's current going into the base of the transistor.
You control the amount of current using a resistor. The amount of current controls how much the transistor opens (if it's in the linear region where it restricts the amount of current going through it, or if it's fully open, acting like a switch allowing a lot of current to go through it)

By default, pins are digital out, they output 0 or 1, or 0v and 5v (or whatever voltage you power the microcontroller with).
Some microcontrollers have DAC modules, which convert a value (for example a 10bit number, a value between 0 and 1023) into a voltage (usually between 0 and an upper voltage, which is usually the configured value of a voltage reference, like [1/2/4] x 1.024v or 1.25v or 2.5v, it varies with microcontroller series). So using a DAC you could output a voltage as close as possible to 0.6v as the dac permits.

If there's no DAC or you don't want to use that functionality, another possibility is to use PWM and a low pass filter (RC on the output pin) to filter the output.. here's an example : http://www.instructables.com/id/Anal...WM-to-Voltage/
and here's a more advanced example (4th order low pass filter with 12bit pwm for a MSP430 microcontroller) :